1. Field of the Invention
The present invention relates to an information recording medium, and more particularly to an information recording medium comprising a disc-shaped resin substrate and a flexible disc-shaped resin film joined thereto and a recording layer enclosed with the combination of the substrate and resin film.
2. Description of Prior Art
Information recording media utilizing a beam of high energy density such as a laser beam have been developed in recent years and are now put into practical use. Such recording medium is generally called "optical disc", and the practical applications thereof have been found, for example, as a video disc and an audio disc as well as a disc memory for a large-capacity computer and a large-capacity static image file.
The optical disc basically comprises a disc-shaped transparent substrate of plastic material or glass material and a recording layer provided thereon. On the surface of the substrate where the recording layer is to be formed may be provided an undercoating layer or an intermediate layer made of polymer material to improve smoothness of the surface of the substrate and adhesion between the substrate and the recording layer and/or to enhance the sensitivity of the resulting optical disc.
Since the recording layer is readily susceptible to external influences, the recording layer is required to be protected from those influences. For the reason, a disc-shaped flexible resin film is covered over the substrate having the recording layer on the recording layer-side to protect the recording layer, and the technique for joining therewith is disclosed in Japanese Patent Provisional Publication No. 59(1984)-36339. The resin film is generally joined to the substrate on its inner-periphery side and outer-periphery side of the substrate, and thereby a sealed space is formed on the recording layer provided between the inner-periphery side and the outer-periphery side of the substrate.
In the optical disc having the above structure, air within the sealed space is likely expanded by variation of external atmospheric pressure or external temperature, whereby a tension is given to the resin film to cause unfavorable phenomena on the substrate such as warp or deformation of the substrate.
As an optical disc having the above-mentioned sealed space therein, an optical disc of air-sandwich structure is conventionally known. With respect to the optical disc of air-sandwich structure, there has been proposed a method of providing a venting hole for allowing air to freely flow (or pass) between the sealed space and the outside in order to adjust difference of atmospheric pressure therebetween. Accordingly, also in the optical disc having the above-mentioned flexible resin film as a protective layer, it can be assumed that a non-joining portion may be locally provided in the joining area between the substrate and the resin film to form a venting hole for adjustment of difference of atmospheric pressure between the inner space and the outside to prevent the above-mentioned unfavorable phenomena such as warp or deformation of the substrate. However, in the case of using the resin film as a protective layer, it is not easy to form a venting hole of suitable size in the known joining procedure. In more detail, when a venting hole is too large, external contaminants such as dust easily enter the inner space through the hole, and hence the recording layer is apt to be deteriorated or damaged. On the contrary, when the venting hole is too small, the aimed function for adjusting the atmospheric pressure is reduced.